MALDI-MS evaluation of poly(3-hexylthiophene) synthesized by chemical oxidation with FeCl3

Poly(3-hexylthiophene) synthesized by chemical oxidation with FeCl3 was inv estigated with matrix-assisted laser desorption/ionization mass spectrometr y (MALDI-MS) using either dithranol or the electron-transfer matrices terth iophene and anthracene. Two to three series of ions were observed in the MA LDI mass spectra. These series all have hexylthiophene repeat groups, and t he end groups/modifications were identified as two or more chlorine atoms t hrough the analysis of isotopic distributions and exact mass measurements. The chlorine atoms may be end groups (on alpha- or beta -carbons of termina l thiophenes) or substituents on the beta -carbons of the thiophene groups along the polymer backbone. Although poly(3-alkylthiophenes) synthesized by this method have been reported to contain iron or chlorine impurities, thi s is the first demonstration that the polymeric product actually contains c hlorine atoms that are covalently bound to the polymer chain (vs residual F eCl3). The FeCl3 oxidizing agent-not the chlorinated solvent-is found to be responsible for halogenating the polymer, as determined by experiments wit h FeBr3 as the oxidant. In addition, we have demonstrated that when using e ither dithranol or the electron-transfer matrices, the ion series represent radical molecular cations, Mcircle+, rather than the protonated molecules or salt adducts that are typically observed in XL LDI-MS.